It is standard to provide, for instance, a two- or three-wheel tractor of the type the operator walks behind with a transmission offering several forward speeds and at least one reverse speed. Such a tractor can include or carry a mower, sickle-bar, or the like, can be used to pull a cart, or can push a blade or cultivator. The several forward speeds are needed for the various tasks the machine performs, which are done at different speeds depending on terrain and other factors, and the reverse is normally only needed occasionally, for instance for backing out of a shed or maneuvering in very tight places.
As a rule reverse is controlled by a separate level which protrudes from the transmission adjacent the standard gear-change lever and which must be actuated and held so long as the machine is backing up. This is quite inconvenient when, as is often the case, the machine is in an inconvenient location like on the side of a steep slope, as it requires that the operator take one of his or her hands off the steering wheel or handle-bars of the device. What is more the operation of disengaging the clutch, typically operated by another handle or pedal, then pulling or pushing the reverse lever and holding it in the actuated position, and then reengaging the clutch takes times so that if, for instance, the machine is on a slope it can start to roll during the reversal operation unless the operator also manages to engage the brake.
In some systems the reversal is obtained by switching a simple direction-reversing gear set into the gear train. This has the advantage that it gives as many reverse speeds as there are forward speeds, but has the concomitant disadvantage that it makes possible a very high and potentially dangerous reverse speed.